Most metals are obtained by removing those metal values from the ores in which they are found in the ground. Once the ore has been mined, the metal must then be separated from the remainder of the ore. One method to separate the metal from the ore is known as leaching. In general, the first step in this process is contacting the mined ore with an aqueous solution containing a leaching agent which extracts the metal from the ore into solution. For example, in copper leaching operations, such as, for example, in the heap or dump leaching of copper ores, sulfuric acid in an aqueous solution is contacted with copper oxide and sulfide minerals of the ore. During the leaching process, acid in the leach solution is consumed and copper is dissolved thereby increasing the copper content of the aqueous leach solution.
The aqueous leach solution containing the leached metal can then be treated via a known process referred to as solvent extraction wherein the aqueous leach solution is contacted with a nonaqueous solution containing a metal-specific extraction reagent. The metal-specific extraction reagent extracts the metal from the aqueous phase into the non-aqueous phase. During the solvent extraction process for copper and certain other metals, the leaching agent is regenerated in the aqueous phase. In the case where sulfuric acid is the leaching agent and the metal is copper, sulfuric acid is regenerated in the aqueous phase when copper is extracted into the organic phase by the extraction reagent. Normally, for every ton of copper removed from the leach solution about 1.5 tons of sulfuric acid is re-generated in the leach solution.
After solvent extraction, the leach solution, now depleted in metal and enhanced in acid, is recycled back to the leaching process to dissolve more metal. The leach solution that exits the solvent extraction plant is called the raffinate. The solvent extraction process typically recovers some 80 to 95% of the metal in the leach solution. Thus the raffinate contains about 5-20% of the leached metal. (The raffinate is recycled back to the leach process and provides the bulk of the leach solution used in the leaching processes.)
In heap and/or dump leaching, fresh ore when leached will produce a leach solution relatively high in metal, often called a high grade leach solution, while ore that has been under leach for some time will produce a leach solution much lower in metal, often called a low grade leach solution. In most operations a leach solution from a new heap or dump will be considered high grade until it has decreased to a certain metal content and it will then be considered a low grade leach solution. A given heap or dump will remain under leach for an additional time producing a low grade leach solution until the amount of metal being leached is so small that it is no longer economical to continue to leach.
It is understood to one skilled in the art that the metal content of a leach solution is usually highest when the ore under leach is the freshest and that the longer the ore is under leach the lower the metal content of the leach solution produced from this ore. See FIGS. 1 and 2.
Often the high grade and low grade leach solutions are combined to produce a combined leach solution containing the desired metal at a concentration lower than the high grade solution and higher than the low grade leach solution. This combined leach solution is then treated in a solvent extraction plant as described above. For the purposes of this application, ore which is producing a high grade leach solution will be referred to as “fresh ore” while ore that is producing a low grade leach solution will be referred to as “partially leached ore”. See FIG. 3.
Alternatively, the leaching process can be carried out in a different way. The low grade leach solution is taken to an intermediate leach solution (ILS) pond from which it is then distributed over fresh ore to give a high grade leach solution. This high grade leach solution is then treated in a solvent extraction process.
When the leaching process on the ore is complete, the ore is removed from the leaching system, that is, raffinate is no longer distributed over the ore and after a short drain down period of time leach solution is no longer collected from the ore. However, when ore is drained down, a significant amount of aqueous solution remains in the ore. After drain down, the leached ore can be moved to a new location or the leached ore can be left in place. If the leached ore is left in place, fresh ore may or may not be placed on top of the leached ore.
In the case where the leached ore is moved from the leach system to a new location, the metal contained in this ore and the metal contained in the leach solution remaining in the leached ore after drain down is lost unless a secondary leach system is put in place. If a secondary leach system is put in place, it is usually not until several or even many years after the leached ore has been relocated. Thus, the metal left in the leached ore is not recover for some significant period of time.
In the case where the leached ore is left in place and new ore is not put on top of the leached ore, the metal contained in the leached ore is lost unless the leached ore is later once again leached.
In the case where fresh ore is placed on top of leached ore and the surface of the leached ore is not made impermeable to solution penetration from the fresh ore leach solution, the total volume of live solution in the leaching system will increase and, as a result, the total metal being recycled will increase over time. Thus, the inventory of leached metal in the leaching system will increase with time.
In the various heap and dump leaching alternatives described above, all or a portion of the raffinate is distributed over the partially leached ore. Thus the amount of metal remaining in the leached ore when the leached ore is taken out of the leaching system depends to a great extent on the metal content of the raffinate. The higher the metal concentration in the raffinate, the more metal that is left in the leached ore after it is drained down and removed from the leaching system. The lower the metal content of the raffinate, the lesser the amount of metal that is left in the leached ore after it has been drained down and removed from the leaching solution.
In the case where fresh ore is placed on leached ore, the lower the metal concentration of the raffinate the lower the copper concentration of the live leaching solution and, therefore, the lower the total metal in inventory in the leaching system.
The metal left in leached ore can have significant economic value because the incremental cost to produce this metal is very low since the cost to mine and leach the metal has already been expended.
When ore is placed for heap or dump leaching, the ore particles placed for leaching vary in size from virtually powder to ⅜ inch and greater depending on the grade of the ore. Metal on the surface of the ore is leached quickly and immediately is taken into solution. Metal inside the larger particles needs to migrate from the inside the larger particles to the surface of the particle before it can be taken into the active flow of leach solution and subsequently recovered by solvent extraction. The process of the metal migrating from the inner portion of a particle of ore to the surface through pores in the ore is called diffusion. The rate of diffusion or metal migration is dependent on the difference in the metal concentration between the leach solution moving through the ore under leach and the metal concentration of the aqueous leach solution trapped in the pores of the particles of ore. The greater the metal concentration of the leach solution trapped in the pores of the ore particles relative to the metal concentration in the leach solution moving through the ore, the greater the rate of migration of metal from the inner part of the particles of ore under leach to the surface of the ore under leach. Thus, the lower the metal concentration of the raffinate, the lower the metal concentration of the solution in the heap or dump and the faster the leached metal migrates from the inner part of the ore particles to the surface of the ore particles. The faster the metal migrates, the faster the leaching will take place and the more complete the leaching will be.
Accordingly there exists a need for an improved process for metal leaching and solvent extraction wherein the metal recovery across solvent extraction and leaching is increased with little or no added capital cost when compared to the leach/solvent extraction systems commonly used today.